Neodymium glass laser having room temperature output at wavelengths shorter than 1,060 nm

ABSTRACT

A laser device including an optically resonant laser cavity formed by dichroic reflectors which are reflective at 920 nanometers and which suppress laser emission at 1,060 nanometers. The device contains a glass host doped with a laserable quantity of neodymium ions in a low concentration (viz. 0.1 to 3 wt percent) which results in the glass exhibiting a ratio of fluorescent intensity peaked at approximately 920 nanometers over the fluorescent intensity peaked at approximately 1,060 nanometers of at least 0.4 as measured by a Cary Model 14 spectrophotometer. The combination of cavity and host results in a laser device which is capable of emitting light energy by stimulated emission at room temperature in a waveband with an optical center at about 920 nanometers.

United States Patent 91 Snitzer et al.

[111 3,711,787 [451 Jan. 16, 1973 [75] Inventors: Elias Snitzer, Wellesley; Charles C. Robinson, Sturbridge, both of Mass; Richard F. Woodcock, Woodstock, Conn.

[73] Assignee: American Optical Corporation, Southbridge, Mass.

[22] Filed: March 10, 1971 [21] Appl.No.: 122,724

[52] U.S. Cl. ..33l/94.5 [51] Int. Cl ..H0ls 3/16 [58] Field of Search ..33 l/94.5; 252/3012, 301.3

[56] References Cited UNITED STATES PATENTS 3,471,409 10/1969 Lee et a1 ..33l/94.5

OTHER PUBLlCATlONS I Hirayama et al., The Effect of Neodymium Environment on its Absorption and Emission Characteristics in Glass. Phys. and Chem. of Glasses Vol. 5. No. 2 (Apr. 1. 1964) pp. 44-51.

Primary Examiner-William L. Sikes Attorney -william C. Nealon, Noble S. Williams and Robert .1. Bird [57] ABSTRACT A laser device including an optically resonant laser cavity formed by dichroic reflectors which are reflective at 920 nanometers and which suppress laser emission at 1,060 nanometers. The device contains a glass host doped with a laserable quantity of neodymium ions in a low concentration (viz. 0.1 to 3 wt percent) which results in the glass exhibiting a ratio of fluorescent intensity peaked at approximately 920 nanometers over the fluorescent intensity peaked at approximately 1,060 nanometers of at least 0.4 as measured by a Cary Model 14 spectrophotometer. The combination of cavity and host results in a laser device which is capable of emitting light energy by stimulated emission at room temperature in a waveband with an optical center at about 920 nanometers.

8 Claims, 4 Drawing Figures PUMP LIGHT iiii NEODYMIUM GLASS LASER HAVING ROOM TEMPERATURE OUTPUT AT WAVELENGTHS SHORTER THAN 1,060 NM BACKGROUND OF THE INVENTION For many applications it is considered desirable to have a laser device capable of producing an output of laser light energy at wavelengths of 920 nanometers. The desirability of generating light at this wavelength is notable with systems utilizing detectors since there are detectors available which are extremely sensitive at this wavelength. Crystals exhibiting such emission are known. For example, a YAG crystal laser is described in an article entitled Oscillation and Doubling of the 0.946 1. Line in Nd*:YAG which appeared in Applied Physics Letter, Vol. 15, No. 4, August 1969, page I 1 l. A problem, however, with the YAG laser is that it is a crystal and thus does not possess the numerous advantages that are known to be attendant with glass lasers.

Glass has various characteristics which can make it an ideal laser host material. It can be made in large pieces of diffraction-limited optical quality, e.g., with an index refraction variation of less than one part per million across a 2.5-cm diameter. In addition, glass lasers have been made in a variety of shapes and sizes from fibers a few microns wide supporting only a single dielectric waveguide mode, to rods 2 meters long or 7.5 cm in diameter. Furthermore, pieces of glass with quite different optical properties can be fused to solve certain system design problems.

Glass composition can be tailored to give an index of refraction in the range of 1.5 to 2.0. Also, thermally stable laser cavities can be achieved by adjusting glass constituents to create an athermal laser glass.

There are two important differences between glass and crystal lasers. First, the thermal conductivity of glass is considerably lower than that of most crystal hosts. The second important difference between glass and crystal lasers is the inherently broader absorption and emission lines of ions in glass. These broader lines imply greater pump-light absorption, greater energy storage and much reduced spontaneous self-depletion for a given energy storage.

A glass laser has been suggested which exhibits an output at about 9180A from neodymium active ions. Such a laser device is described in U.S. Pat. No. 3,270,290 by R. D, Maurer. In connection with room temperature operation, it is well known that it is desirable to utilize a laser at room temperature in order to eliminate cumbersome'equipment otherwise necessary to cool the laser device.

SUMMARY OF THE INvENTION In accordance with the present invention a glass neodymium doped laser device is provided which is capable of emitting radiation at about 920 nanometers at room temperature (approximately C).

Accordingly, it is an object of the present invention to provide a neodymium doped glass laser device which is capable of operating at room temperature and which will generate laser light energy in a waveband with an optical center located at about 920 nanometers.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an emission curve showing the fluorescent emission properties of glasses utilized in laser devices of the present invention;

FIG. 2 is a schematic representation of the various energy levels in a Nd ion;

FIG. 3 is a diagrammatic illustration of a laser device of the present invention;

FIG. 4 is a transmittance and reflectance curve of a reflector useful in the laser device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, a laser device is provided which is comprised of a neodymium doped glass laser host positioned within an optically regenerative laser cavity. It has been found that trivalent neodymium ions in glass hosts typically have emission curves of the general shape shown in FIG. 1. This curve is provided at the outset to illustrate properties which are useful in carrying out the Object of the present invention.

The fluorescent curves shown were measured in a Cary 14 spectrophotometer by placing the glass sample in a copper fixture which in turn was placed in the sam ple compartment of the Cary. The glass was irradiated at right angles with a Xenon arc lamp through a filter which blocked the transmission of wavelengths longer than approximately 800 nm. The fluorescent spectrum was recorded using the automatic slit control which adjusted the slit width so that the output of a coiled tungsten filament lamp with a filament temperature of approximately 2,800K produced a constant deflection on the recording chart for all wavelengths. Thus the recording chart must be corrected to obtain the true relative intensities by dividing the chart deflection by a factor proportional to the energy radiated by the tungsten lamp at the wavelengths of interest. We have estimated the correction factor for obtaining the ratio of the 920nm fluorescent intensity to the 1,060nm intensity to be approximately unity. This estimate was made by using the tungsten emissivities measured by J.C. DeVos (J.C. DeVos, Physics 20,690 (1954) for a ribbon filament tungsten lamp operating at 2,800K in a calculation of the energy radiated by the coiled filament lamp at the two wavelengths of interest. The intensity ratios reported here were measured directly from the Cary charts using no correction factor.

In FIG. 1 a curve 10 is shown with peaks 12 and 14 at 920 nanometers and 1,060 nanometers respectively. In connection with these and similar peaks, it is to be understood that the actual range of useful fluorescent emission is somewhat broad. In fact, in accordance with the invention peak 12 can have a band width of 20 nanometers located between 905-925 nanometers, as is represented by spectral region A of FIG. 1, while peak 14 can have a width of 20 nanometers located between 1,050-l ,070 nanometers, as is represented by spectral region B of curve 10. Although curve 10 shows other peaks, for purposes of the present invention the entire peaks represented by spectral regions A and B of curve 10 are the most critical. Numerous tests have indicated that when a-neodymium doped glass host is positioned in a cavity with reflectors that suppress emission at 1,060 nanometers, peaks A and B are the only peaks that need be considered in evaluating whether the laser will emit at 1,060nm or 920nm at room temperature. Although a ratio of A/B of at least 0.4 produces operative results, that is, laser action at 920 nanometers at room temperature (20C), it is to be understood that in accordance with the invention the greater the magnitude of the foregoing ratio the more effective will be the host for producing the desired laser emission when positioned in the cavity of the present invention.

As indicated above, in addition to considering the emission spectra of the host glass, consideration must also be given to the optically regenerative laser cavity into which the host glass is positioned. In accordance with the invention the reflectors forming the laser cavity must suppress laser emission at 1,060 nanometers. It is to be understood that such reflectors are available and that the reflectors per se form no part of the present invention. For example, dichroic reflectors are available which transmit approximately 85 percent of the light at 1,060 nanometers while reflecting approximately 99.7 percent of the light between the range of 800-1 ,000 nanometers.

Although not intended to be restricted to a particular theory, an understanding of the energy level scheme including the 1 manifold of the Nd ion is useful in explaining the present invention. In this regard, FiG. 2 is provided as a schematic representation of the various energy levels in a Nd ion.

In accordance with the invention, room temperature laser emission at 920 nanometers has been found to occur with the laser operating as a partial three level laser. Thus, from the spectra and from the correlation of the band separations with an octahedral crystal field, the energy levels shown in FIG. 2 are assumed present where the "I manifold level 21 has a degeneracy (3 of 4 and contains 2 Kramers doublets. Laser emission at 920 nanometers (line 20 FIG. 2) is considered to terminate at one or both of these doublets. Since the total degeneracy for the 1 level is known to be 10 with level la considered as a Kramers doublet (g ,,=2), which is the lowest degeneracy possible for any level with an odd number of electrons in the absence of a magnetic field, level 1 must have a degeneracy (g equal to 4.

A condition necessary for laser action surrounding to this invention is that the population of the initial state he at least as large as the terminal state which requires, therefore, that the initial state population be at least 0.033 of the total population in the ground manifold 912)- The cavity losses for the 1,060 nanometers emission must be higher than those for the 920 nanometers emission including the effect of the population in level 2.

In accordance with the invention, a laser device was constructed and tested. The host glass had the composition in percent by weight as set forth in Example 16 below. From the laser glass of Example 16, a rod 3 inches in length was fabricated utilizing known techniques. The rod 30 was positioned in an optically regenerative laser cavity formed by reflectors 32 and 34, as is shown in FIG. 3. To demonstrate the laser device of the present invention, two extreme experiments were conducted. In the first experiment a reflector R (32 of FIG. 3) which was 98 percent reflective for light at 920 nanometers and 98.4 percent reflective at 1,060 nanometers, was employed as one reflector with a second reflector R (34 of FIG. 3), which was 99.5 percent reflective at 920 nanometers and percent reflective at 1,060 nanometers. With the combination of reflectors R and R laser action at 920 nanometers was observed with the host glass of Example 16. In a second test with both reflectors 32 and 34 being R types, laser emission at 920 nanometers occurred even more readily.

The foregoing tests, as well as other tests, proved that when the ratio of A/B discussed above is greater than 0.4, the laser device will generate laser light energy at about 920 nanometers at room temperature (20C) if the device includes reflectors compatible with that wavelength and which suppress laser emission at 1,060 nanometers. A pump light source is not shown in FIG. 3, it being understood that many pump sources are available which will produce the required population inversion in the neodymium ion. One such pump source commonly employed is a xenon flash tube. In this regard, the hardware for producing energy inversions are conventional and form no part of the present invention.

The transmittance and reflectance curve of the R type reflectors is shown in FIG. 4 of the drawing. Such a reflector is available from Spectra-Physics, 1250 West Middlefield Road, Mountain View, Calif. 94040.

The laser glass as set forth in Example 16, as well as the other examples, are preferably formed in the following manner. The alkali earths and alkaline earth metals are added to the batch as nitrates or carbonates and all other constituents of the finished glass (silica, neodymium, zinc, boron, antimony) are added directly as oxides. The constituents are added in the known stoichiometric amounts to yield a glass having a final composition as set forth in the various examples. The glass making raw materials must be of high purity and, in particular, must be free of contamination from iron or other elements which would cause light absorption at the desired emission wavelength if they were present in the finished glass. The finished glass, for example, should not contain more than 5 parts per million ofiron as Fe O The glass may be prepared by fusing the raw materials in a ceramic crucible heated in a Globar" electric furnace. No special atmosphere is necessary in the furnace. The raw materials are mixed intimately and as completely as possible in a mixing device that does not introduce any contamination. The mixed batch is loaded into a high purity ceramic crucible which will not contaminate the melt with undesirable impurities. The crucible should be at a temperature of approximately 2,700F when the raw material is charged, the loading operation taking approximately 2 hours since the level in the crucible drops as the batch materials fuse together to form the glass and thus require the addition of more batch. When the charging of the batch is completed, the temperature of the melt is raised to approximately 2,800F and is held at this temperature for 1 hour to free the melt of striae. The temperature of the glass is then lowered to approximately 2,700F where it is maintained for a period of about 1 hour before casting. The temperature value last recited is suitable for a melt of 1 lb. but it is to be understood that the preferred temperature at casting is a function of the size of the cast with larger casts requiring lower temperatures for control of the glass. The glass may be cast in a cast iron mold, and is transferred to an annealing oven just as soon as it has cooled down slowly overnight to room temperature.

In the following examples:

Column 1 components in finished glass Column 2 percent by finished glass weight of components in the Column 3 fluorescence intensity peaked between 905 nanometers to 925 nanometers fluorescence intensity peaked between 1,050 nanometers to 1,070 nanometers (referred to in the specification as A/B) Column 4 figure of the frawing showing relative fluorescence intensity curve Sin C5 0 ZnO CaO B110 B203 Sb O N6 0.

CaO z a CaO 1 1,

EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 EXAMPLE 6 EXAMPLE 7 EXAMPLE 8 1 2 3 CaO 41.1 .634 A00, 49.0 Sr() 7.9 Nd,0 2.0

EXAMPLE 9 1 2 3 N320 4.15 .555 K 0 2.10 CaO 32.80 A1 0 43.50 MgO 1.45 13110 5.50 La O 5.85 S10, 2.65 Ndl O 2.00

EXAMPLE 10 1 2 3 8110 26.6 .42 A1 0 8.8 (51:0 63.6 Nd q 1.0

EXAMPLE 1 l 1 2 3 8510 59.0 .45 A1 0;. 7.8 GeO 32.2 Nd Q 1.0

EXAMPLE 12 1 2 3 C5 0 68.9 .68 TiO 29.1 Nd Q, 2.0

EXAMPLE 13 1 2 3 C5 0 69.1 .408 13 O 27.9 Nd 0 3.0

EXAMPLE 14 1 2 3 C5 0 52.4 .505 GeO 45.6 I Nd O 2.0

EXAMPLE 15 1 2 3 C5 0 61.6 .412 B 0 35.4 Nd O, 3.0

EXAMPLE 16 1 2 3 S10 36.57 .47 C5 0 46.58 ZnO .71 C210 1.42 8:10 11.23 13,0 2.22 S12 0 .61 15111 0 .66

population to lase at 920 nanometers. From the foregoing examples and numerous tests, O.l-3 wt. percent of Nd O in the final glass results in a laser glass which is usable in accordance with the invention. The best results, however, result from a glass containing 0.5-1.5 wt. Nd O In accordance with the invention, operative results occur when the neodymium ion concentration is kept low, as is the case with the foregoing examples. lt has also been discovered that with Nd-silicate glass lasers, heavy monovalent alkali ions, that is, potassium, rubidium and cesium, when included in the glass to replace lighter alkali ions, that is, sodium and lithium, improve the overall results. That is, the ratios of fluorescent intensity peaked at 920 nanometers over the fluorescent intensity peaked at L060 nanometers (A/B) greatly in excess of 0.4 are possible. Thus, A/B ratios of silicate glasses containing neodymium in the range of O.l to 3 wt. percent as laser active ingredient are substantially increased by the use of these heavier alkali monovalent ions.

With respect to the divalent ions, it has been discovered that an increase in their concentration tends to lower the A/B ratio. It is to be emphasized, however, that as long as the neodymium concentration is kept below 3 wt. percent, usable results will occur with any of the known prior art Nd doped las'er glasses. However, heavier divalent ions such as lead, cadmium and strontium have been found to lower the foregoing ratio to a lesser extent when compared to the effect caused by light divalent ion such as calcium. Barium has been especially desirable in increasing the foregoing ratio in silicate glasses with trivalent neodymium within the range of 0. l-3 wt. percent. In this regard, usable glasses may include barium oxide in the range of -10 wt. percent with approximately wt. percent being preferred. A range of 0-10 wt. percent is suitable for other divalent ions usable in the glass composition.

Tests show that in silicate glass compositions containing more than approximately l0 weight percent of alkali, a given molar percentage of Cs O is superior than the same molar percentage of Rb O which in turn is better than the same molar percentage of K 0 insofar as an increase of A/B is concerned. From the standpoint of maximizing the A/B ratio of a laser component, it would be desirable to have all of the alkali provided by use of cesium or rubidium in relatively large weight percentages ofthe order of percent.

Composition ranges for Nd-silicate base glasses which promote high *A/B ratios are given in the tables below:

TABLE I Range in by wt. Constituent in finished glass divalent ion lead oxide 0 l0 monovalent ion potassium oxide 7 2O laser ion Nd O .l 3 base silicate balance TABLE 2 Range in by wt. Constituent in finished glass divalent ion lead oxide 0 l0 monovalent ion rubidium oxide 7 20 laser ion M 0. .1 3 base silicate balance Constituent divalent ion monovalent ion laser ion base Constituent divalent ion monovalent ion laser ion base Constituent divalent ion monovalent ion laser ion base Constituent divalent ion monovalent ion laser ion base Constituent divalent ion monovalent ion laser ion base Constituent divalent ion monovalent ion laser ion base Constituent divalent ion monovalent ion laser ion base Constituent divalent ion monovalent ion laser ion base TABLE3 lead oxide cesium oxide Nd,0 silicate TABLE 4 cadmium oxide potassium oxide Ndg03 silicate TABLE 5 cadmium oxide rubidium oxide Ma e, silicate TABLE 6 cadmium oxide cesium oxide Ndzoa silicate TABLE 7 barium oxide potassium oxide Nd' O silicate TABLE 8 barium oxide rubidium oxide Nap,

silicate TABLE 9 barium oxide cesium oxide Nd Q, silicate TABLE 10 strontium oxide potassium oxide Nd,0

silicate TABLE l l strontium oxide rubidium oxide Nd O silicate Range in by wt.

in finished glass 0 l0 balance Range in by wt.

in finished glass 0 l0 balance Range in by wt.

in finished glass 0 l0 balance Range in '71 by wt.

in finished glass 0 l0 balance Range in by wt.

in finished glass 0 l0 balance Range in by wt.

in finished glass 0 l0 balance Range in "/0 by wt.

in finished glass 0 l0 balance Range in by wt.

in finished glass 0 l0 balance TABLE 12 t fei bft if Constituent in finished glass divalent ion strontiumgxide 10 monovalent ion potassium oxide 7 .2 0? laser ion Nd zgg 1; 5 7 base silicate balance TABLE 13 Range in by wt. Constituent in finished glass divalent ion lead oxide 0 monovalent ion mixture of two or more 7 20 of potassium, cesium and rubidium laser ion Nd O .l 3 base silicate balance TABLE 14 Range in by wt. Constituent in finished glass divalent ion cadmium oxide 0 l0 monovalent ion mixture of two or more 7 20 of potassium, cesium and rubidium laser ion Nd O .1 3 base silicate balance TABLE 15 Range in by wt. Constituent in finished glass divalent ion barium oxide 0 10 monovalent ion mixture of two or more 7 of potassium, cesium and rubidium laser ion M1 0, .1 3 base silicate balance TABLE 16 Range in by wt. Constituent in finished glass divalent ion strontium oxide 0 10 monovalent ion mixture of two or more 7 20 of potassium, cesium and rubidium laser ion Nd O .l 3 base silicate balance It should be understood that the term silicate base as used throughout this specification and claims is diameter were fabricated. For this range of concentrations the threshold for laser action varied from 100 to 137 joules input respectively.

Verification that these rods were lasing at 0.92 pm and not 1.06 pm was made using an infrared image converter and appropriate narrow band pass filters for the wavelengths in question. Also, laser emission at 0.92 pm was verified by use of a spectrograph.

When the laser rod is to be utilized in a laser system where solarization is a problem, the rod containing the low concentration of Nd O mag be appro riately clad to prevent the solarrzatron of t e laser ro It is to be dicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

We claim:

1. A laser device capable of generating laser light in a waveband with an optical center located about 920 nanometers at room temperature, said laser device comprising a neodymium doped glass host which exhibits a ratio of fluorescent intensity peaked at approximately 920 nanometers over fluorescent intensity peaked at approximately 1,060 nanometers of at least about 0.4 and means for suppressing laser emission at l ,060 nanometers wherein neodymium is present in the glass host in a maximum amount of 3 weight percent of the oxide.

2. The device as set forth in claim 1 wherein said means for suppressing laser emission at 1,060 nanometers are dichroic reflectors which are more reflective at 920 nanometers than at 1060 nanometers.

. 3. The device as set forth in claim 2 wherein said dichroic reflectors form an optically resonant laser cavity for light having an optical center at 920 nanometers.

4. The laser device as set forth in claim 1 wherein the neodymium ion concentration of said neodymium doped host is 0.1 3 wt. percent of the oxide.

5. The laser device as set forth in claim 1 wherein the neodymium ion concentration in said neodymium doped host is 0.5 1.5 wt. percent of the oxide.

6. The laser device as set forth in claim 1 wherein said neodymium doped glass host consists essentially of a silicate base glass wherein the monovalent alkali ions in said silicate glass are selected from 7 20 wt. percent of an oxide of the group consisting of potassium, rubidium, cesium, and mixtures thereof and trivalent neodymium oxide within the range of approximately 0.1 -3 wt. percent.

7. The laser device as set forth in claim 6 wherein the divalent metal oxide content of said silicate glass is within the range of approximately 0-10 wt. percent and wherein the metal is selected from the group consisting of cadmium, lead, barium and strontium.

8. The laser device as set forth in claim 7 wherein said divalent metal oxide is approximately 5 wt. percent barium oxide. 

2. The device as set forth in claim 1 wherein said means for suppressing laser emission at 1,060 nanometers are dichroic reflectors which are more reflective at 920 nanometers than at 1060 nanometers.
 3. The device as set forth in claim 2 wherein said dichroic reflectors form an optically resonant laser cavity for light having an optical center at 920 nanometers.
 4. The laser device as set forth in claim 1 wherein the neodymium ion concentration of said neodymium doped host is 0.1 -3 wt. percent of the oxide.
 5. The laser device as set forth in claim 1 wherein the neodymium ion concentration in said neodymium doped host is 0.5 -1.5 wt. percent of the oxide.
 6. The laser device as set forth in claim 1 wherein said neodymium doped glass host consists essentially of a silicate base glass wherein the monovalent alkali ions in said silicate glass are selected from 7 - 20 wt. percent of an oxide of the group consisting of potassium, rubidium, cesium, and mixtures thereof and trivalent neodymium oxide within the range of approximately 0.1 -3 wt. percent.
 7. The laser device as set forth in claim 6 wherein the divalent metal oxide content of said silicate glass is within the range of approximately 0-10 wt. percent and wherein the metal is selected from the group consisting of cadmium, lead, barium and strontium.
 8. The laser device as set forth in claim 7 wherein said divalent metal oxide is approximately 5 wt. percent barium oxide. 